Preparation and molecular structure of a dinuclear rhodium complex having an unbalanced (15 + 17)-electron count

Article abstract of DOI:10.1016/j.jorganchem.2004.06.064

On stepwise reaction of [Rh₂(acac)₂{μ-C(p-tol) ₂}₂(μ-SbiPr₃)] (1) with PMe₃ and CO the unsymmetrical dinuclear complex [Rh₂(acac) ₂(PMe₃){μ-C(p-tol)₂}₂(μ-CO)] (3) is formed, which owing to the X-ray crystal structure contains the CO in a bridging and the phosphine ligand in a terminal position. The reaction of [Rh₂(acac)₂{μ-C(p-tol)₂}₂(μ- SbiPr₃)] (1) with PMe₃ led to the displacement of the stibine by phosphine and afforded the highly labile intermediate [Rh ₂(acac)₂(PMe₃){μ-C(p-tol)₂} ₂] (2) in which the PMe₃ ligand possibly occupies a terminal position. Treatment of 2 with CO at low temperatures gave the unsymmetrical dinuclear complex [Rh₂(acac)₂(PMe ₃){μ-C(p-tol)₂}₂(μ-CO)] (3), the molecular structure of which was determined crystallographically.

Full text of DOI:10.1016/j.jorganchem.2004.06.064

Journal of Organometallic Chemistry 689 (2004) 4917–4920
www.elsevier.com/locate/jorganchem
Note
Preparation and molecular structure of a dinuclear rhodium
complex having an unbalanced (15 + 17)-electron count
*
Ulrich Herber, Kerstin Ilg, Helmut Werner
Institut fu¨r Anorganische Chemie, Universita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Received 13 February 2004; accepted 25 June 2004
Available online 25 November 2004
Abstract
The reaction of [Rh2(acac)2{l-C(p-tol)2}2(l-SbiPr3)] (1) with PMe3 led to the displacement of the stibine by phosphine and
afforded the highly labile intermediate [Rh2(acac)2(PMe3){l-C(p-tol)2}2] (2) in which the PMe3 ligand possibly occupies a terminal
position. Treatment of 2 with CO at low temperatures gave the unsymmetrical dinuclear complex [Rh2(acac)2(PMe3){l-C(p-
tol)2}2(l-CO)] (3), the molecular structure of which was determined crystallographically.
ꢀ 2004 Published by Elsevier B.V.
Keywords: Rhodium; Acetylacetonato complexes; Bridging carbene ligands; Trimethylphosphine complexes; Ligand substitution
1. Introduction
or PPh3 failed [3], we found in the continuation of these
studies that under strictly controlled conditions the sti-
bine-containing precursor [Rh2(acac)2(l-CPh2)2(l-Sbi-
Pr3)] reacts with PMe3 by bridge-ligand exchange and
formation of [Rh2(acac)2(l-CPh2)2(l-PMe3)] [4]. This
complex, which is also accessible on an alternative route
[4,5], has been characterized crystallographically. In this
communication, we illustrate that the reactivity of the
related starting material [Rh2(acac)2{l-C(p-tol)2}2(l-
SbiPr3)] (1) towards trimethylphosphine is somewhat
different and that on stepwise treatment of 1 with
PMe3 and CO a dinuclear rhodium complex with an
unbalanced (15 + 17)-electron count is accessible.
Recently, we reported that the stibine-bridged com-
pounds [Rh2(acac)2(l-CR2)2(l-SbiPr3)] (R = Ph, p-tol)
react with Lewis bases L such as CO, CNtBu and SbEt3
by displacement of the triisopropylstibine ligand and
formation of analogous dinuclear rhodium complexes,
in which the ligand L like SbiPr3 in the starting material
occupies a bridging position [1,2]. In contrast, if the pre-
cursors [Rh2(acac)2(l-CR2)2(l-SbiPr3)] were treated
with PiPr3 or PPh3 not only a substitution of SbiPr3
for PiPr3 or PPh3 occurred but also, most unusually,
the migration of one acetylacetonato ligand from one
metal center to the next. Following this route, a series
of mixed-valence Rh(0)–Rh(II) complexes were gener-
ated for which there was no precedence [1,2].
2. Results and discussion
After our attempts to prepare analogous mixed-va-
lence compounds ½ðPR30 ÞRhðl-CR2Þ RhðacacÞ ꢀ with
Addition of excess trimethylphosphine to a solution
of compound 1 in pentane at ꢁ50 ꢁC leads, after warm-
ing to room temperature, to the precipitation of a red
solid which correctly analyzes as [Rh2(acac)2(P-
Me3){C(p-tol)2}2] (2) (Scheme 1). While 2 is thermally
quite stable and only moderately air-sensitive, it de-
composes in solution almost instantaneously. Thus, no
2
2
phosphines that are sterically less demanding than PiPr3
*
Corresponding author. Tel.: +49 931 888 5270; fax: +49 931 888
4623.
E-mail address: helmut.werner@mail.uni-wuerzberg.de (H.
Werner).
0022-328X/$ - see front matter ꢀ 2004 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2004.06.064

4918
U. Herber et al. / Journal of Organometallic Chemistry 689 (2004) 4917–4920
iPr
iPr
iPr
H3C
H3C
CH3
CH3
Sb
O
O
O
O
R
R
PMe3
- SbiPr3
[Rh2(acac)2(PMe3)(µ-CR2)2]
Rh
Rh
C
C
2
R
R
1
CO
O
C
H3C
H3C
PMe3
O
R
R
O
CH3
Rh
Rh
C
C
O
O
R
R
H3C
3
Scheme 1.
1
reliable H or 31P NMR spectrum of 2 in C6D6 or ace-
tone-d6 could be obtained. The 31P CP/MAS NMR
spectrum displays a single resonance at d = 5.7 ppm,
which is split into a doublet indicating that the phos-
phine ligand probably occupies a terminal and not a
bridging position. The 13C CP/MAS NMR spectrum
of 2 shows two signals for the CO and equally two sig-
nals for the CH carbon atoms of the acetylacetonate
units. This illustrates that the two acac ligands are spec-
troscopically and presumably also stereochemically
different.
Taking into consideration that the CO-bridged com-
plex [Rh2(acac)2{l-C(p-tol)2}2(l-CO)] was already
known [2], we studied the reactivity of intermediate 2 to-
wards carbon monoxide. When we passed a slow stream
of CO through a solution of 2 in acetone, rapid decom-
position occurred. To avoid this, we first prepared a sat-
urated solution of CO in acetone, cooled the solution to
ꢁ50 ꢁC and then added a sample of 2 to this solution.
After warming to room temperature and evaporation
of the solvent, a red microcrystalline solid of the analyt-
ical composition corresponding to 3 was isolated in 78%
yield. In contrast to 2, compound 3 is stable in solution
and can be stored under argon for weeks without
decomposition. The IR spectrum of 3 shows a strong
absorption at 1822 cmꢁ1, i.e. in a region where the
stretching modes for doubly bridging carbonyl ligands
should appear. The 31P NMR spectrum of 3 displays a
doublet-of-doublet resonance at d = ꢁ5.2 ppm with a
large (129.7 Hz) and a small (7.6 Hz) 31P–103Rh
coupling.
Fig. 1. Molecular structure of compound 3; anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen atoms
and carbon atoms of the p-tolyl rings are omitted for clarity.
trigonal–bipyramidal with C(1), C(3) and O(2) in the
equatorial and C(2) and O(1) in the axial positions. De-
spite the dissymmetry, the two bond lengths between the
metal centers and the carbonyl carbon atom C(1) are
nearly the same (Table 1). However, the distances be-
tween both Rh(1) and Rh(2) and the carbene carbon
atoms C(2) and C(3) differ significantly as was also
found in the unsymmetrical stibine-bridged compound
[Rh2(acac)Cl(l-CPh2)2(l-SbiPr3)] [2]. The dissymmetry
of the molecule 3 is also reflected in the unequal bond
lengths between Rh(2) and the oxygen atoms O(1) and
O(2) of the corresponding acetylacetonate unit. The dis-
˚
The X-ray crystal structure analysis of 3 illustrates
(see Fig. 1) that indeed the CO group occupies a bridg-
ing and the PMe3 ligand a terminal position. The coor-
dination geometry around Rh(1) is distorted octahedral
while that around Rh(2) is best described as distorted
tance Rh(1)–Rh(2) of 2.5163(6) A differs only slightly to
that of the symmetrical complex [Rh2(acac)2(l-
˚
CPh2)2(l-CO)] (2.4933(9) A) and lies in the range of
other dinuclear rhodium(I) compounds with a metal–
metal bond [6–9].

U. Herber et al. / Journal of Organometallic Chemistry 689 (2004) 4917–4920
4919
Table 1
ipso-C of p-tol), 137.4, 134.6 (both s, para-C of p-tol),
132.5, 128.6, 127.9, 122.7 (all s, ortho-C and meta-C of
p-tol), 96.7, 95.7 (both s, CH of acac), 29.1 (s, CH3 of
acac), 24.7 (s, PCH3), 21.2 (s, CH3 of p-tol). 31P CP/
MAS NMR (162.0 MHz): d 5.7 [d, J(Rh,P) = 119.6
Hz]. Found: C, 58.98; H, 5.54; Rh, 22.87%.
˚
Selected bond distances (A) and bond angles (ꢁ) with estimated SD for
compound 3
Bond distances
Rh(1)–Rh(2)
Rh(1)–P
2.242(3)
2.4011(15)
1.947(5)
2.135(4)
2.173(5)
1.958(5)
Rh(2)–C(2)
Rh(2)–C(3)
Rh(1)–O(3)
Rh(1)–O(4)
Rh(2)–O(1)
Rh(2)–O(2)
2.074(5)
2.018(4)
2.147(3)
2.169(3)
2.156(3)
2.098(3)
Rh(1)–C(1)
Rh(1)–C(2)
Rh(1)–C(3)
Rh(2)–C(1)
3: A solid sample of 90 mg (0.10 mmol) of 2 was
added to 20 ml of acetone, which was cooled at ꢁ50
ꢁC and saturated with CO. The solution was slowly
warmed to r.t. and stirred for 1 h. The solvent was evap-
orated in vacuo and the residue recrystallized from 15
ml of pentane. After the solution was stored at ꢁ20
ꢁC for 12 h, red crystals precipitated which were washed
with small amounts of pentane (ꢁ20 ꢁC) and dried in va-
cuo; yield 70 mg (78%), m.p. (dec.) 108 ꢁC. Anal. Calc.
for C44H51O5PRh2: C, 58.94; H, 5.73; Rh, 22.95. IR
Bond angles
Rh(1)–C(1)–Rh(2)
Rh(1)–C(2)–Rh(2)
Rh(1)–C(3)–Rh(2)
C(1)–Rh(1)–C(2)
C(1)–Rh(2)–C(2)
C(2)–Rh(1)–C(3)
C(2)–Rh(2)–C(3)
80.2(2)
C(2)–Rh(2)–O(1)
P–Rh(1)–C(1)
P–Rh(1)–C(2)
176.93(16)
96.40(16)
96.59(14)
175.01(11)
85.44(12)
84.16(13)
132.89(4)
73.42(14)
73.70(16)
81.04(18)
82.3(2)
P–Rh(1)–C(3)
O(3)–Rh(1)–O(4)
O(1)–Rh(2)–O(2)
P–Rh(1)–Rh(2)
88.34(17)
94.37(13)
(C6H6): m(CO) 1822, m(COacac) 1584, 1516 cmꢁ1 1H
.
NMR (C6D6, 200 MHz): 7.53, 7.13 (both m, 8H,
ortho-H of p-tol), 7.02, 6.91, 6.73, 6.62 [all d,
J(H,H) = 8.4 Hz, 8H, meta-H of p-tol), 5.46, 4.96 (both
s, 2H, CH of acac), 2.22, 2.12, 1.98, 1.26 (all s, 12H, CH3
of acac), 1.91, 1.79 (both s, 12H, CH3 of p-tol), 0.76 [d,
J(P,H) = 9.1 Hz, 9H, PCH3]. 31P NMR (C6D6, 81.0
In summarizing, the present investigations have
shown that the size of the entering PR3 ligand has a sig-
nificant influence on the course of the reaction of the
starting material 1 with trialkylphosphines. While upon
treatment of 1 with the bulky PiPr3, apart from the dis-
placement of the bridging stibine ligand, an unusual
intramolecular rearrangement occurs, involving the
migration of one acetylacetonato ligand from one rho-
dium center to the next, the reaction of 1 with the less
bulky PMe3 proceeds along a different route. Although
the initially formed product 2 is rather labile, the 13C
CP/MAS NMR spectrum as well as the subsequent
addition of CO confirms that also in the intermediate
the two Rh(acac) fragments have been preserved. De-
spite the fact that the above-mentioned Rh(0)–Rh(II)
complexes ½ðPR03ÞRhðl-CR2Þ RhðacacÞ ꢀ are quite stable
1
3
MHz): ꢁ5.2 [dd, J(Rh,P) = 129.7, J(Rh,P) = 7.6 Hz].
Found: C, 59.26; H, 5.41; Rh, 22.21%.
Crystal data for 6: Crystals were obtained from pen-
tane at ꢁ20 ꢁC. C44H51O5PRh2, Mr = 896.64; triclinic,
˚
ꢀ
˚
space group P1 (no. 2), Z = 2, a = 10.7783(18) A,
˚
b = 13.895(3) A, c = 14.795(2) A, a = 78.38(2)ꢁ,
3
˚
b = 74.634(19)ꢁ, c = 74.42(2)ꢁ, V = 2037.7(6) A ,
Dcalc = 1.461 g cmꢁ3, k = 0.71073 A, T = 173(2) K,
˚
l(Mo Ka) = 0.892 mmꢁ1. Crystal size 0.21 · 0.19 ·
0.18 mm3; 2Hmax = 50.06ꢁ; 21,424 reflections were meas-
ured, 6772 of these were independent (Rint = 0.0649) and
employed in the structure refinement (481 parameters).
2
2
[2], attempts to transform 3 into the mixed-valence com-
pound [(PMe3)Rh{l-C(p-tol)2}2Rh(acac)2] failed.
The
R values are R1 = 0.0412 and wR2 = 0.0947
[I > 2r(I)] and R1 = 0.0590 and wR2 = 0.1015 (all data);
reflex/parameter ratio = 14.1; min/max residual electron
density: 0.927/ꢁ1.461 e Aꢁ3. Data were collected on a
˚
3. Experimental section
IPDS diffractometer from Stoe.
A semi-empirical
The experiments were carried out under an atmos-
phere of argon by Schlenk techniques. The starting
material 1 was prepared as described in the literature
[2]. Melting points were measured by differential thermal
analysis (DTA). Abbreviations used: s, singlet; d, dou-
blet; m, multiplet.
2: A solution of 120 mg (0.12 mmol) of 1 in 15 ml of
pentane was cooled at ꢁ50 ꢁC and treated dropwise with
60 ll (0.60 mmol) of trimethylphosphine. The solution
was slowly warmed to r.t. and stirred for 30 min. A
red solid precipitated, which was separated from the
mother liquor, washed three times with 10 ml of pentane
each and dried in vacuo; yield 96 mg (92%), m.p. (dec.)
102 ꢁC. Anal. Calc. for C43H51O4PRh2: C, 59.46; H,
5.92; Rh, 23.69. 13C CP/MAS NMR (100.6 MHz): d
186.7, 183.9 (both s, CO of acac), 151.3, 145.2 (both s,
absorption correction was applied. The structure was
solved by direct methods (SHELXS-97) [10] and refined
against F2 by least-squares (SHELXS-97) [11]. All non-
hydrogen atoms were refined anisotropically. The posi-
tions of all hydrogen atoms were calculated according
˚
to ideal geometry (distance C–H 0.95 A) and used only
in structure factor calculations.
4. Supplementary material
Crystallographic data (excluding structure factors)
for the structure analysis have been deposited with the
Cambridge Crystallographic Data Centre, CCDC No.
232920 for compound 3. Copies of this information
may be obtained free of charge on application to the

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U. Herber et al. / Journal of Organometallic Chemistry 689 (2004) 4917–4920
(b) U. Herber, B. Weberndo¨rfer, H. Werner, Angew. Chem., Int.
Ed. Engl. 38 (1999) 1609.
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: +44 1223 336033; e-mail: deposit@ccdc.
cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
[2] U. Herber, T. Pechmann, B. Weberndo¨rfer, K. Ilg, H. Werner,
Chem. Eur. J. 8 (2002) 309.
[3] U. Herber, Dissertation, Universita¨t Wurzburg, 2000.
¨
[4] T. Pechmann, C.D. Brandt, H. Werner, Chem. Eur. J. 10 (2004)
728.
Acknowledgements
[5] (a) T. Pechmann, C.D. Brandt, H. Werner, Angew. Chem. 112
(2000) 4069;
We thank the Deutsche Forschungsgemeinschaft
(SFB 347) and the Fonds der Chemischen Industrie
for financial support, the latter in particular for a
Ph.D. grant (to K.I.). Moreover, we are most grateful
to Mrs. R. Schedl and Mr. C.P. Kneis (elemental analy-
sis and DTA) and Dr. R. Bertermann (CP/MAS NMR
spectra).
(b) T. Pechmann, C.D. Brandt, H. Werner, Angew. Chem. Int.
Ed. 39 (2000) 3909.
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[11] G.M. Sheldrick, SHELXL-97, Program for Refinement of Crystal
Structures, University of Go¨ttingen, Germany, 1997.
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